1. Field of the Invention
Embodiments of this invention relate to dual function catalyst compositions for the partial oxidation of alkanes and the simultaneous oxidation of incompletely oxidized carbon oxides and methods for making and using same.
More particularly, embodiments of this invention relate to dual function catalyst compositions for the partial oxidation of alkanes and the simultaneous oxidation of partially oxidized carbon oxides, where the catalyst compositions include at least one compound of the general formula (I):MoVaNbbPtcMdZeOx  (I)where:                a is a number having a value between about 0.15 and about 0.50,        b is a number having a value between about 0.05 and about 0.30,        c is a number having a value between about 0.0001 and about 0.10,        d is a number having a value between about 0.0 and about 0.40,        e is a number having a value between about 0.0 and about 0.10,        x is a number depending on the relative amount and valence of the elements different from oxygen in formula (I),        M is one or more of the following elements: Ag, Te, and Sb, and        Z is one or more element selected from Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Ce, Pr, Nd, Sm, Tb, Ta, W, Re, Ir, Au, Pb, B, and mixtures thereof.        
2. Description of the Related Art
Hydrocarbon conversion reactions are useful industrial processes to convert abundant components of a petroleum feedstock into other compounds having greater industrial utility. Examples of such reactions include the conversion of ethane to ethylene, the conversion of propane to propylene, the conversion of propylene to acrylic acid (AA), the conversion of isobutene to methacrylic acid, the conversion of hexenes to aromatics, or other similar reactions that convert a lower value feedstock into a higher value product. A specific example is the production of acrylic acid from hydrocarbon feedstocks.
Acrylic acid is an important industrial chemical. The global demand for acrylic acid in 2009 was close to 4 million tons per year. The major commercial process to produce acrylic acid is the two-stage oxidation of propylene. In the first stage, propylene is oxidized in the presence of oxygen and steam to acrolein. In the second stage, acrolein is oxidized in the presence of oxygen and steam to acrylic acid. Each stage operates at different optimum temperature and hydrocarbon concentration. Acrylic acid is an important material for the production of many useful products. Acrylic acid undergoes the typical reactions of carboxylic acids, for example esterification with alcohols. The esters and salts of acrylic acid are collectively known as acrylates (or propionates). The most common alkyl esters of acrylic acid are methyl-, butyl-, ethyl-, and 2-ethylhexyl-acrylate. Polymerization of the acids and acrylates results in the commercially important polyacids, polyacrylates and polyalkylacrylates.
Virtually all of the commercially produced acrylic acid is made from the oxidation of propylene. However, oxidation of propane to acrylic acid would be more economical, because propane is a cheaper feedstock than propylene.
Direct propane oxidation to acrylic acid has been investigated for more than two decades as an alternative to the current commercial propylene oxidation process to produce acrylic acid. See, e.g., M. Ai, Journal of Catalysis, 101, 389-395 (1986), and U.S. Pat. No. 5,380,933. So far, propane to acrylic acid has not been commercially realized despite the relative price advantage of the propane feedstock.
In the propane oxidation to acrylic acid, it is often advantageous to operate the process such that propane conversion is limited by the available oxygen. The oxygen concentration is controlled at low concentrations to ensure the feed mixture is below the flammability limit of the reaction mixture; however, limiting the available oxygen results in a limited conversion of propane. For the process to be economical, the unreacted propane must then be recovered and recycled. Also, propylene is produced as an intermediate and is recycled along with propane.
Some of the better performing catalysts for oxidizing propane to acrylic acid (AA) produce carbon monoxide (CO) as a significant byproduct, but the catalysts will not further oxidize CO to carbon dioxide (CO2). EP1930074A1 disclosed propane oxidation catalysts according to the formula (I):MoVaTebNbcZdOx  (I)wherein a=0.0-0.50, b=0.0-0.45, c=0-0.5, d≦0.05, and x is a number depending on the relative amount and valence of the elements different from oxygen in formula (I), and Z is at least one element selected from Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Sb, Ce, Pr, Nd, Te, Sm, Tb, Ta, W, Re, Ir, Pt, Au, Pb, and Bi, provided that at least two different metal species are contained in the catalyst composition, that is, one of a, b, c, and d are not zero.
For reasons of efficiency, high yields of product from starting hydrocarbon material is desirable. One way to improve yield is to recycle unreacted and partially reacted starting material back to the reactor. In propane oxidation to acrylic acid, the unreacted and partially reacted starting materials are propane and propylene. However, the effluent stream also contains the byproduct gases carbon monoxide (CO) and carbon dioxide (CO2). With each pass, CO and CO2 accumulate in the recycle stream to the point at which they reach levels deleterious to the reaction. For this reason, it is important to remove CO and CO2 or otherwise prevent their accumulation in the recycle stream. There are standard processes to remove CO2 from the recycle stream, which are well characterized and relatively inexpensive. For example, CO2 can be scrubbed from the recycle stream by base washing. However, the removal of CO is somewhat more difficult and problematic, and as a result, more costly.
EP2179793 disclosed propane oxidation catalysts according the general formula (I):MoVaXbQcZdOe  (I)wherein X is P, which may be replaced in part by Bi, for instance up to a molar ratio Bi/P of 1/1, Q is at least one of Nb, Ta and W (which includes the combined use of Nb and Ta, Nb and W, and Ta and W, as well as the use of all three elements), a=0.15-0.50, b=0.02-0.45, in particular 0.05-0.40, c=0.05-0.45, d s 0.05 and e is the molar number of oxygen binding to the metal atoms present in this mixed oxide which follows from the relative amount and valence of the metals elements, and Z is at least one element selected from Na, K, Si, Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Ce, Pr, Nd, Sm, Tb, Re, Ir, Pt, Au, and Pb.
WO2006008177 disclosed metal oxide catalysts comprising the metal oxides of Mo, V, Te and Nb, and may optionally contain oxides of other metal elements, as long as these do not adversely affect the function of the resulting material as a catalyst in the oxidation reactions referred to herein. The calcined catalyst material to be leached in the method of the present invention is a material of the average general formula (I):MoVaTebNbcZdOx  (I)wherein a=0.15-0.50, b=0.10-0.45, in particular 0.10-0.40, c 0.05-0.20, d 0.05 and x is a number depending on the relative amount and valence of the elements different from oxygen in formula (I), and Z is at least one element selected from Ru, Mn, Sc, Ti, Cr, Fe, Co, Ni, Cu, Zn, Ga, Y, Zr, Rh, Pd, In, Sb, Ce, Pr, Nd, Te, Sm, Tb, Ta, W, Re, Ir, Pt, Au, Pb, and Bi.
As CO levels increase in the recycle stream, the recycle stream must be purged to reduce CO levels, but these results in concurrent loss of propane and propylene reducing process efficiency as raw material is lost.
The problem of the accumulation of CO in the recycle stream can be addressed in one of two ways. CO can be removed from recycle stream. As discussed previously, this way is costly and difficult. Alternatively, CO production can be suppressed, prevented or eliminated so that CO accumulation in the recycle stream is prevented, suppressed or eliminated in the first instance.
Thus, it would be beneficial for hydrocarbon conversion processes generally, and specifically for the conversion of propane to acrylic acid, to have a means to minimize or eliminate CO accumulation in the recycle stream. The present invention focuses on the second route to CO mitigation and is directed to the preparation of dual function catalysts that combine propane oxidation to acrylic acid functionality with a mild oxidation functionality to convert CO to CO2, which is more easily removed from the recycle stream. The catalyst of the present invention accomplishes this without detrimentally affecting the propane to acrylic acid functionality.